Phosphorus pentafluoride (PF5) is an industrially useful material in the fields of semiconductors and batteries. In these fields, a mixture in a starting material can exert an adverse influence on the performance or safety of the final product. Therefore, high purity is required of the starting material for use in these fields.
Various processes for preparing phosphorus pentafluoride (PF5) have so far been proposed. In order to acquire high-purity PF5 effectively, it is of concern how to separate PF5 from a mixture with a raw material and a by-product associated with the preparation process. In particular, it is an issue how to separate PF5 from a mixture with HF or SO3, which is a starting material of PF5; HCl, POF3, or PF3, which is by-produced in the preparation of PF5; or SO2 or H2S, which is contained in the starting material HF.
Patent Literature 1 below discloses synthesis of PF5 by the reaction between HPF6 with a sulfur-based acid under high pressure. This process involves by-production of fluorosulfuric acid (HSO3F), sulfur trioxide (SO3), HF, and so forth. Separation between PF5 and these by-products needs purification equipment including a large-sized distillation tower.
Patent Literatures 2 and 3 disclose synthesis of PF5 by the reaction between liquid or gaseous HF and phosphorus pentachloride (PCl5). These processes by-produce hydrogen chloride (HCl) gas in an amount five times the equivalent of PF5. Because the boiling point of HCl is −84.9° C., close to the boiling point of PF5 (−84.8° C.), separation by an ordinary distillation technique is difficult, which renders the processes industrially disadvantageous.
As mentioned, preparation of PF5 has the problem that HF, HCl, F2, Cl2, CO2, SO3, SO2, H2S, PF3, POF3, NO2, NO3, NH3, and so on, some of which are the starting materials and some others are by-produced in the reaction, are entrained as a mixture in the produced PF5. Available purification methods are limited, however, because PF5 is highly corrosive and labile in water and air.
Patent Literatures 4 and 5 disclose a method in which an HF carrier gas is brought into contact with a starting material comprising a phosphorus and fluorine atom to extract PF5 in the carrier gas. However, in order to recover and separate HF used as a carrier gas, the method needs condensation equipment using a coolant for cooling to −50° to 0° C.
Patent Literatures 6 and 7 propose processes for separating PF5 and HCl using an ionic liquid. The processes are not economical, however, because of the use of expensive ionic liquids for adsorption and desorption. Moreover, when an HCl concentration is higher than the concentration of PF5, the process is not efficient because it is necessary to remove the large amount of HCl by adsorption to the ionic liquid.
Thus, it has not been easy to obtain high-purity PF5 through general synthesis processes including a purification technique making use of a difference in boiling point or an ordinary gas separation technique by removing a mixture of components contained in the starting materials or components by-produced during reaction using simple equipment.